Table of contents

Be more flexible: extending flexibility of sheet metal forming Seoul, Republic of Korea

International Deep Drawing Research Group annual conference 2020 was virtually held on 26 – 30 October in Seoul, Republic of Korea. The conference was organized by the Korean Society for Technology of Plasticity. The conference chairs were Prof. Myoung-Gyu Lee, Seoul National University, Dr. Daeyong Kim, Korea Institute of Materials Science, Dr. Jung Han Song, Korea Institute of Industrial Technology, and Prof. Ji Hoon Kim, Pusan National University.

List of International Deep Drawing Research Group, Virtual Conference, Topics, Plenary speakers, Tutorial sessions, Participants & Authors, Scientific Committee, Local Organizing Committee, Advisory Board, Acknowledgement, Sponsors and Exhibitors, Editors, Images are available in the pdf.

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□ Number of submissions received: 112

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□ Number of submissions accepted: 91

□ Acceptance Rate (Number of Submissions Accepted / Number of Submissions Received X 100): 81.2

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□ Total number of reviewers involved: 70

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□ Contact person for queries:

Prof. Ji-Hoon Kim, Pusan National University (Email: jhkim@pusan.ac.kr)

Many industrial sheet metal parts experience a non – proportional loading history. The determination of such non – proportional loading paths on a laboratory scale is still very challenging and time - consuming. Most tests require different machines, tools and measurement techniques. Norz and Volk [1] have presented a new approach to create arbitrary non – proportional loading paths by using a cruciform specimen in combination with a draw bead tool. This simple experimental setup lead to a significant reduction in the experimental effort. It allows the generation of arbitrary loading paths by using only one machine, one tool and one specimen geometry without any further processing of the specimen between the forming steps. Nevertheless, the strain path could only be controlled manually after a certain forming step. In this paper, an in-line strain measurement was used to observe the real – time strain path during the experiment. This information was used to create a semi-automatic approach in which the draw bead height is adapted in order to create a prescribed strain path.

Working with individual in-process controllable elements to support an elastic blankholder is the state-of-the-art for stamping irregular shaped sheet metal parts without wrinkles and cracks. It is thus needed to carefully determine the position for each expensive device. This study is therefore aimed to appraise a feasible approach with finite element analysis, in that the blankholder is first modelled as a rigid object instead of an elastic one to evaluate the contact pressure of the blankholder to the blank during the forming process. The sites showing a locally relative high contact pressure, which are usually induced by local thickening of material, can be regarded as the locations of the supporting elements to intently form a local high pressure to control the material flow and thus to eliminate the local thickening there. As a result, since the blankholder force can be efficiently delivered from the supporting elements directly to the blankholder as well as to the blank, it is not only applied to the rectangular box-shaped part but also to a fender-like irregular sheet metal part. Furthermore, the initiated contact pressure can be maintained at locations during the whole stamping process. This is also experimentally validated by stamping a sound fender-like irregular sheet metal part with an elastic blankholder supported by elements located accordingly at those sites suggested by this study.

Among the sheet forming processes, coining is a specific operation which makes it possible to correct shape defects or to perform thickness reductions in the parts. This operation often requires a very high force which is likely to have an impact on the functioning of the tool or the press. It is therefore important that the coining forces be evaluated accurately, but this is not allowed by the existing analytical calculation formulas. The objective of this study is to improve the accuracy of the calculation of the coining forces via the adaptation of an existing formula. Such adaptation was carried out based on a study of the influence of the flat coining parameters. To that end, a series of experimental measurements was performed. An instrumented force measuring setup made it possible to measure the maximum coining force. Several parameters were analysed, including the coining ratio and the sheet thickness. Numerical simulations were carried out at the same time, in order to understand the influence of certain parameters on the coining force. 2D numerical models were developed using the Forge NXT2 software. The simulations and the experimental tests were analysed and the results revealed the influential phenomena which have to be taken into account in the analytical formula, in particular the coining surface and the friction. In order to study the friction more thoroughly, ring compression tests were performed so as to determine the friction coefficients, based on the Coulomb's law limited to Tresca. This type of test is representative of the stresses undergone by the metal during the coining operation. Finally, a new calculation formula is proposed, in order to integrate the coining surface and friction more accurately.

This study aims to analyse the formability characteristics by friction characteristics of automotive steel sheets to improve sheet forming during the stamping process. During the stamping operation, the steel sheet is pushed into the tool surface, a complex phenomenon occurs, and many parameters interact. Therefore, automotive steel researchers want to optimize the friction of automotive steel sheets to improve the forming process, such as drawing ratio limit, dome height limit, cup drawing test, etc. In addition, in this study, the finite element method is used in combination with a numerical solution to present the friction behavior of high strength steel sheets in automobiles, taking into account the operating deformation mechanism during the formability test.

The use of lubricants in forming processes is common because they reduce friction between the tools and workpiece during the forming process. Today, mainly mineral oils are used as conventional lubricant. In order to cut their amount in the future, the friction between tools and workpiece is investigated by using water as lubricant in this work. Therefore, the tools' surfaces are structured with laser induced periodic surface structures (LIPSS) to make them hydrophilic and strip drawing tests are carried out at different traverse velocities to evaluate the resulting friction force. The results show that at a traverse velocity of v = 10 mm/s the maximum friction force is lowest for the structured tool with water (F = 7.0 N) in comparison with unstructured tools in combination with a conventional lubricant. Consequently, the potential of water as lubricant combined with a hydrophilic surface structure is highlighted.

Stress based forming limit diagram (SFLD) and polar effective plastic strain (PEPS FLD) diagram are two techniques reported to be independent of strain path change. Thus, these techniques are capable of predicting failure in pre-strained sheet metal samples. Therefore, in this study evolution in anisotropy properties of sheet metals were considered in terms of evolution in yield shape during conversion of conventional strain based forming limit diagram (FLD) to SFLD and PEPS-FLD. The evolution in anisotropic properties were modelled as a function of effective plastic strain depending on which the anisotropic coefficients of non-quadratic yield model Yld2000-2d were calculated. Further, to analyse the strain path change effect, metal sheets of extra deep drawing quality steels were first deformed under biaxial condition in Marciniak in-plane stretch forming setup. The pre-strained samples were subsequently deformed with hemispherical punch till the necking appears. For failure prediction, finite element modelling of the complete procedure was performed considering evolution in the yield shape. Finally, both the failure criteria were implemented for failure prediction. Implementation of the evolution in yield function in finite element analysis and failure criteria provided better results.

Strain path change is a typical phenomenon during continuous stamping operations of sheet metal for a variety of applications including automotive body parts. During stamping, a punch continuously deforms a metal sheet to produce a desired geometry while following various strain path transitions depending on overall design of the stamping process. The strain path change can potentially alter the expected forming limit of the material. Previous researchers investigated the effect of changing strain path by loading sample in two distinct steps. Typically, between the steps the sample is unloaded before being re-loaded in the new strain path. This practice reflects the key challenge in elucidating this strain path dependent deformation, which is the ability to control the strain path change in a single deformation stage in an experimental set-up. In this work, a novel testing rig and specimen geometry that is capable of changing the strain path of a sample continuously without unloading the specimen were conceptualised, modelled and subsequently manufactured. Using this apparatus, the specimen was deformed in the uniaxial strain path in the first step before being deformed biaxially without unloading in between the steps. Thus, the apparatus ensures that the sample undergoes a continuous strain path change without unloading between the steps. The size of this mechanical test rig permits it to be placed inside a scanning electron microscope (SEM) chamber in order to study strain path transition in-situ to highlight strain localization and related microstructural changes in real time. Utilizing this test set-up, strain path change and corresponding strain values along each strain path were evaluated. The changes in material microstructure were concurrently investigated using in-situ SEM and electron back scattered diffraction (EBSD) analysis.

In recent times, lightweight design and functional integration in metal forming and especially in deep drawing operations lead to complex geometries. These geometries can cause variable defects in production. Especially with modern materials like AHSS or UHSS, deep drawing becomes more challenging. One common way to meet these challenges and manufacture sheet metal parts without defects is the application of drawbeads in the forming tool. Drawbeads are used for an exact control and forecast of the material flow during the deep drawing process. It was already shown, that different material parameters like the tensile strength and the fracture strain are changed significantly after running through a drawbead. In addition, there are various indications that the tribological system in a drawbead passage is also changed significantly and that this has further influence on the ongoing drawing process.

This assumption is examined with an experimental setup derived from industrial deep drawing processes. Therefore, different sheet metal materials are drawn through a drawbead geometry under the same conditions like the blank holder force, the drawing velocity or lubrication. After the drawbead passage, 3D surface parameters are measured and compared to the values in its initial state. Following, conventional strip-drawing tests with preloaded strips are carried out. The variation of the friction coefficient after a drawbead passage is analysed and compared to the initial state. The results are correlated with the surface values. For these investigations, three different materials are used: a conventional deep drawing steel because of its wide application as well as an advanced high strength steel AHSS and an aluminium alloy of the 6xxx series as representatives for modern lightweight materials.

Deviations of the properties of semi-finished sheet metal parts affect the finished part's geometry and its properties. With knowledge of the deviations, forming processes can be adjusted to a certain degree in order to maintain the part quality. Typically, in mass production, the sheets are blanked before deep drawing. Within the blanking processes, the necessary cutting force and cutting stroke are both influenced by the properties of the material. Thus, by the analysis of the process data, which is obtained in blanking processes, deviations of the properties of semi-finished parts could be determined. Within the scope of the work, it is analyzed to which degree process data, which is gained during blanking processes, can be used for the determination of the properties of semi-finished parts and their deviations. For this purpose, a blanking process is investigated for two different batches of the dual phase steel DP600 with a nominal sheet thickness of 2.0 mm and varying mechanical properties. In addition, the sheet thickness of the specimens is reduced by 5 and 10 % for one batch. The recorded process data from the blanking process is correlated with the properties of the sheet materials, which are determined with uniaxial tensile tests.

Reducing weight of Body in white (BIW) is a well-known solution to tackle environmental issues in automotive industry. Replacing steel by lighter aluminium sheet material is an accepted alternative. However, this trend brings challenges like the forming of Aluminum into complex car body parts. These challenges signify the necessity of more advanced tools to obtain defect-free parts. Nowadays, it is a common practice to implement forming simulations at the early stage of part design in order to identify critical features and to optimize the production process. Although, it is known that friction and lubrication conditions are one of the most influential sources affecting product quality in aluminium sheet metal forming, it is currently not considered in detail in stamping simulations. The current application case focuses on the role of friction and lubrication modelling in stamping simulations of an aluminium hood inner part of the new Ford Transit. A comparison was made between different friction models to identify and resolve split issues in the forming simulations. Simulation results were used to guide try-out towards efficient and successful elimination of these issues on the physical panels. Finally, different possible scenarios corresponding to production conditions were investigated to define a robust process window.

IN718 is a nickel-based super alloy that retains its mechanical properties at elevated temperatures and hence has numerous applications in the aerospace industry. In this study, deep drawing of IN718 sheet at elevated temperatures (700-1000°C) has been investigated experimentally. Initially, stress-strain response with failure strain at different temperatures was evaluated by uniaxial tensile tests along different orientations. Subsequently, hot deep drawing and re-drawing tools were designed and fabricated. A circle-grid marked circular blank of 3mm thickness was used in the first draw to form a circular cup followed by re-drawing (2nd draw). The major and minor diameter of the circular grid on the blank after the deep drawing was used to examine the strain distribution in the drawn cup and further used to calculate the thickness strain. These deep drawing operations were carried out at different temperatures with a constant blank holding force. Further, the deep drawing force at different temperatures was examined. The effect of the test temperatures during hot deep drawing and the process parameters on the variation of the thickness strain over the surface of the drawn cup has been studied. Finally, the major and minor strains are plotted in Keeler and Arcelor V9 forming limit curve to examine the formability of IN718 sheets at elevated temperature.

A comparative investigation on springback prediction of aluminum stampings has been completed using an industry-type "shotgun" die as the benchmark case. 5xxx aluminum alloy at a thickness of 1.5 mm was used for the shotgun die tryout. After stamping tryout, the formed panel was scanned to capture its sprung geometry. The studied aluminum alloy was characterized by means of uniaxial tensile test and hydraulic bulge test to determine the material isotropic hardening behavior and planar anisotropy. In order to determine the material kinematic hardening behavior, the exclusive Yoshida model fitting test was conducted in tension-compression (T-C) and compression-tension (C-T) loading modes with the one-cycle and three-cycle loading schemes. The shotgun benchmark case was simulated using FEA software LS-DYNA®. By comparing the simulation results with the tryout measurements, the accuracy of springback prediction was examined to evaluate the influence of material modeling, loading mode and loading cycle in the Yoshida model fitting test. Based on the current benchmark study, the best practice in springback prediction and compensation of aluminum stampings and in conducting the Yoshida model fitting test of aluminum alloys were summarized for industrial applications.

The aim of present work is to develop a crystal plasticity modeling approach to integrate slip, dynamic recrystallization (DRX) and grain boundary sliding (GBS) for simulating the deformation and texture evolution of magnesium alloys deformed at the high temperature. A GBS model is developed to evaluate strain and grains' rotation induced by GBS, and implemented into the polycrystal plasticity framework VPSC. The VPSC-DRX-GBS model can well reproduce the stress-strain curves and texture evolution. The calculated GBS contribution ratio in tension is obviously higher than in compression due to easier cavity nucleation on grain boundaries under tensile creep-aging conditions. The significantly different texture development in tension and compression deformation due to GBS is well reproduced by the proposed model.

The homogeneous anisotropic hardening (HAH) model formulation, which is solely based on distortional plasticity, was improved recently. In this study, a robust coefficient identification scheme was developed for this updated model. Tension-compression tests were conducted on a mild steel sheet sample to assess the hardening behavior in the forward-reverse loading mode. An inverse identification procedure was employed to calibrate the coefficients of the model. The Nelder-Mead simplex and genetic optimization methods were investigated and the results regarding the accuracy of the calibration were compared. As a result, both optimization methods led to reasonable coefficients for the new model.

Process stability and facility security are the two main purposes for measuring the force in sheet metal forming facilities. These two purposes look for guaranteeing the good health of the facility avoiding overloads and at the same time look for variations in the process force that could lead to products out of the demanded quality. Traditionally there have been different approaches to measure the force exerted by the press. Piezoelectric sensors and strain gages are the most widely used technologies. However there are some common limitations that reduce the success in the measurement of the force by the aforementioned sensors: force sensors tend to have a signal drift and the introduction of the sensors together with the electronics represents an extra cost that in some cases can be high. As an alternative to the implementation of force sensors, soft sensing approaches take advantage of the already available signals in the facility to estimate non measured variables, the force in this case. The present work aims at predicting the force curve that a servo press exerts in forming processes reading the internal servomotor signals. The approach has been evaluated with several forming processes achieving an error lower than 3% for all the cases.

The martensitic transformation in quenching and partitioning (Q&P) steels is greatly influenced by loading conditions such as environmental temperature and loading speed, and thus impacts the macroscopic mechanical properties during the deformation process. Within this work, an elastic-visco-plastic self-consistent (EVPSC) framework coupling with phase transformation model is used to simulate the stress strain responses as well as the microstructure evolution of the multi-phases Q& P980 steel under uniaxial tension process. A temperature and strain rate dependent transformation kinetics is incorporated into the model and phase transformation behaviors of the Q& P980 steel under different temperatures (25°C∼100 °C) and strain rates (0.0002s-1∼2s-1) are successfully characterized. The corresponding stress strain responses under different loading conditions are predicted and compared with the experimental data.

Due to a low density and high tensile strength, the aluminum alloy EN AW 7075 T6 offers a high lightweight potential for structural components. Since its formability is limited at room temperature in the T6 temper state, the potential of this alloy for automotive bodies is only utilizable by adapted deep drawing processes. In recent years, process chains suited for warm and hot forming have been researched and developed. However, warm and hot forming solutions require additional process steps and a complex tooling system in comparison to cold forming processes. Alternatively, the forming of such blanks at room temperature in the W temper state is favorable since conventional tools can be used. The W temper state is a heat treatment condition achieved after solution heat treatment and subsequent quenching, which is characterized by an increased ductility. However, this condition is unstable, due to the onset of natural ageing. With increasing time after the quenching step, the strength of the material increases, which leads to a reduction of formability. Another phenomenon that occurs after quenching is the Portevin Le-Chatelier effect. This effect causes the formation of flow lines during cold forming and results in a decrease of ductility. Hence, the objective of the investigations was to determine the formability of EN AW 7075 as a function of the natural ageing time after solution heat treatment and quenching. Therefore, tensile tests of various aged samples were carried out. The results show a relation of the formability to the natural ageing time and a dependency on the quenching rate. Furthermore, a heat treatment strategy for EN AW-7075 was developed, that considers manufacturing processes like the cathodic dip coating. The influence of the quenching rate, ageing time and temperature as well as the influence of temperature of the paint baking process after the cathodic dip coating were considered. Therefore, a design of experiments and tensile tests were carried out. Thus, the deep drawing of EN AW-7075 at room temperature is particularly promoted.

This paper aims to experimentally investigate the effects of lubrication and temperature on the tribology behavior of a magnesium alloy, AZ31B, sliding against 86CrMoV7. The effects of lubricant additives, including oil film improver and extreme pressure (phosphorous donors), on the friction coefficient and surface morphology in the sliding zone were studied. It was found that the relatively lower surface hardness of the material significantly affects the wear of the materials in the sliding zone. The lubricant with oil film improver and extreme pressure can reduce friction. Temperature rise brings about lower friction due to the lubricant additives.

Steel-polymer composites have low density, high specific flexural stiffness and vibration damping properties over other metal materials. However, the weak interfaces between steel and polymer sheet are the typical locations where the failure occurs. Furthermore, interfacial failure is a fatal reason of the degradation of mechanical properties. Therefore, predicting interfacial failure of the final product is an important technology in commercialization of steel-polymer composites. In this study, we built forming limit diagram (FLD) based on two different failure modes: failure of the bottom skin steel and interfacial failure between the skin steel and core polymer. Acoustic emission (AE) technique was conducted to observe the interfacial failure moment. Peak frequency was found to be the key feature to distinguish interfacial failure. Peak frequency band that indicates interfacial failure was from 300 kHz to 500 kHz. This peak frequency band made it possible to build FLD considering the interfacial failure.

Developed to increase the energy efficiency of automobiles, steel-polymer sandwiched composites are attracting attention due to their lightweight and multi-functionality and are being actively researched recently. In order to apply those composites to industrial fields, their formability must be investigated in order to prevent the degradation of mechanical properties of the composites during the forming process in various shapes. Therefore, experimental studies have recently been conducted to evaluate the formability of the composites. However, there are few studies to predict the formability of the composites considering the mechanical properties of core polymer and interfacial properties. In this study, formability tests were carried out for evaluating the forming limit diagram of the composites. Also, the formability test of the composites was simulated considering the viscoplastic properties of the core polymer using Arruda-Boyce model and the interfacial properties using the cohesive zone model. Finally, the effect of the interfacial adhesion on the formability of the composites was investigated, from which an optimal condition was explored for the interfacial adhesion strengths that can ensure the formability of the composites for specific applications.

In this study, the deformation performance of galvanized deep drawing steel is investigated depending on coating thickness and forming limit of non-alloy pure zinc coated steel. Regarding to this; 100 and 150 g/m² Zn-coated samples are used to see the relevance with finite element analysis. To determine the deformation condition of Zn coating the samples were prepared at different elongation levels, then the samples are examined with surface views and cross-sectional analyses with scanning electron microscope to observe the amount of deformation at the onset of crack in the zinc coating cross-section. The change of zinc coating with elongation at cross-section region of the material will be investigated via microstructural analyses. It is aimed to determine the most suitable zinc coating level depending on the deep drawing amount in interstitial free (IF) steel usage. Furthermore, the performance of zinc coating will be investigated with finite element analyses with the goal of validating experimental results. Depending on the results of this study, the influence of zinc coating for a physical component will be carried out as a new study.

This paper presents a fuzzy modelling technique for the prediction of surface scratching problems encountered in sheet metal forming processes involving contact sliding. To generalize the contact conditions with die-sheet interactions in such processes, ball-on-disk sliding investigations, including modelling and experiment, were carried out with various ball diameters to mimic the die-contact radii effect and a range of contact loads to simulate the effect of contact stress variation experienced in forming processes. Because die-contact radius, surface roughness and contact stress changes cannot be exactly defined in a practical production process, in the modelling of the ball-on-disk configuration, ball diameter, normal load, surface roughness, sliding cycles and extent of scratching depth in a metal sheet surface were treated as fuzzy variables for the prediction. Based on the comparison between predictions with the relevant experimental measurements, it can be seen that the fuzzy modelling technique developed in this paper can predict very well the surface scratching.

To predict the spring back after hot stamping of sheet metals, a user material routine incorporated with phase transformation effects has been developed and implemented in general-purpose commercial finite element software. The mechanical response of the metallographic structure incorporates variation of transformation plastic strain and transformation volume expansion. Additionally, the thermal response incorporates the dependencies of thermal properties and latent heat of phase transformation. The start timing of diffusive transformation was determined using Scheil's law, and the Kolmogorov–Johnson–Mehl–Avrami equation was adopted for the progression of diffusive transformation. The Koistinen and Marburger equation was used for the progression of martensite transformation. Using the developed model, we considered the effects of phase transformation on shape fixability. We conducted experiments that focused on measuring the shape fixability of U-bend hot formed parts. In this study, we show that the spring back of the U-bend parts increases under conditions in which phase transformation occurs before formation has been completed. Furthermore, the spring back could accurately predict by using the developed model.

The equibiaxial expansion test allows the evaluation of the mechanical behaviour of materials on a large strain range. In this context, a bulge test device was developed which enables a fast heating stage, a uniform heating of the specimen, and the control of the temperature during the expansion phase. Although during the test, the temperature is uniform at the pole of the cap, the contact with the tools imposes a gradient along its radius. Therefore, a thermo-mechanical model of the bulge test was built in order to improve knowledge about the influence of the temperature gradient in the evaluation of the hardening behaviour at the pole of the cap. Numerical simulations of the bulge test were performed considering both isothermal and anisothermal conditions. The anisothermal conditions considered reproduce the temperature gradient observed experimentally. This enables the analysis of the impact of the thermal gradient in the stress versus strain evolution at the pole of the cap and the comparison with experimental results.

With the increasing implementation of industry 4.0 technologies in production plants, issues such as control and monitoring as well as feedback control of plants and processes are becoming more and more important in current development trends. Thereby, potential approaches to such issues are generally accompanied by an increased use of sensors in production systems, aimed at gaining operation-, product- and process-specific data. In this respect, this paper focusses on the shear cutting process of a tool-bound punching machine. One of the most important parameters for the evaluation of shear cutting processes is the cutting force required. However, the precise measurement of this highly dynamic process parameter proves to be relatively challenging. The limited installation space within the tools and the loads caused by highly dynamic axis movements decisively restrict the options for sensor-based process monitoring. In order to face these challenges during sensor implementation in such punching machines, issues regarding measuring concept, localization and resulting measurement deviation of the sensors have to be solved simultaneously. In this paper, relevant measurement concepts and their properties are discussed, and the most favourable integration possibilities are derived. Based on this, several measurement concepts are identified that could be suitable for the application considered. These measuring methods include hydraulic pressure sensors, strain gauges or piezoceramics. The major contribution of this paper is an experimentally conducted systematic comparative study of these methods, in which the accuracy of the sensors used is evaluated and the influence of different integration positions on the measurement result is investigated.

A three-dimensional (3D) crystal plasticity finite element method (CPFEM) is developed to investigate the effect of grain boundary strength on heterogeneous strain partitioning in FCC polycrystals. The proposed method incorporates electron backscatter diffraction (EBSD) maps into finite element analyses. The numerical analysis accounts for crystallographic texture, its evolution and 3D grain morphologies. Furthermore, grain boundaries are also mapped with a special finite element framework that allows material properties to be assigned to the grain boundaries. The material parameters and the grain boundary strengths are obtained by calibration to experimental uniaxial tension curves for single and polycrystals. Numerical simulations of uniaxial tension are performed and the effects of grain boundary strength on the onset of non-uniform deformation is investigated. The predicted local strain evolution is compared with corresponding experimental results from digital image correlation (DIC) measurements of an AA5754 aluminium sheet. The results showed that an excellent agreement was reached when the grain boundary properties were set so that the hardness was five times that of the average polycrystal response.

In this study, the perturbation approach for predicting material's forming limit strains under non-associated flow rule (non-AFR) is proposed. The influence of yield function and plastic potential function on the forming limit curve (FLC) evaluated by the perturbation approach are discussed through analyzing the normalized growth rate of a perturbation. In the framework of non-AFR, Hill'48 and Yld2000-2d are chosen for AA5754-O. The results show that the left side of FLCs predicted with the different forms of yield function and plastic potential nearly overlap. Hence, it is concluded that the yield function and plastic potential have a negligible influence on the forming limit strain under a negative strain path. However, the FLC under a positive strain path is principally dependent on the relationship between strain ratio β and stress ratio α, which can be determined by the plastic potential. Additionally, in comparison to the FLC under AFR, an increase in the forming limits strain is observed near the plain strain region, when the derivative of the normalized yield function concerning α is positive and vice versa.

This study aims at investigating on the descriptions of stress-strain response and the assessment of formability for Al-Cu-Mg alloy under impact hydroforming (IHF). IHF process effectively combines the advantages of the flexible liquid and the impact impulse. The formability of low plastic materials can be increased by using IHF process. The elongation of Al-Cu-Mg alloy can even be more than 50 % compared with the quasi-static forming means (27 %∼30 %) if the strain rate was on the level of 5000/s. The stress-strain curve is characterized with S-shape. It is found that the strain hardening rate decreases linearly at the stage III, increases linearly at the stage IV and decreases non-linearly at the stage V. The deformation mechanism is the interaction of dislocation accumulation and dynamic recovery under the transmission from non-sheared to sheared precipitation when the strain rate is on the level of 3000/s∼5000/s. A modified Kocks-Mecking model was established to describe the mechanical property of AA 2B06. In addition, the high strain rate formability was evaluated by the new means which revealed the relationship between the impact energies, deep drawing height ratios and the deep drawing ratios by using the finite element modelling of the solid liquid coupling.

In this paper, a pressure-coupled Drucker function is proposed to model the plastic deformation and fracture from shear to plane strain tension of AA5182 sheet. Experiments are conducted for AA5182 in shear, uniaxial tension and plane strain tension, and the force-stroke curves are measured during the tests. The plastic deformation is modeled by the Drucker function. The Drucker function is modified to consider the pressure effect for fracture limit stress prediction of the alloy. The pressure-coupled Drucker function for plasticity and fracture is calibrated by an inverse engineering approach. The calibrated plasticity models are then applied to numerical prediction of plastic deformation of the alloy under various loading conditions. It is observed that the pressure coupled Drucker function accurately describes the plastic deformation under large plastic deformation as well as the onset of ductile fracture under these loading conditions.

In this study, an aluminium alloy of AA5182 is taken as the research object to study strain hardening under large plastic deformation. Tensile tests are done for four specimens, including dog-bone specimens, notched specimens, specimens with a central hole and in-plane shear specimens. Bulging tests are also conducted to measure strain hardening under balanced biaxial tension. In addition, an experimental method called in-plane torsion test is also used for shear loading. At least three experiments are completed for each type of specimens along the rolling direction (RD), diagonal direction (TD), and transverse direction (DD). The stroke of each tests is measured by a digital image correlation (DIC) system, and the load-stoke curves were obtained for the tests. Combined with an inverse engineering method, the strain hardening properties are calibrated for the alloy under different loading conditions of shear, uniaxial tension, plane strain tension, and balanced biaxial tension. The strain hardening under various loading conditions is compared and modelled by various yield functions to evaluate their performance. It is concluded that inverse engineering approach is a simple but powerful method to obtain the stress-strain curve up to large plastic deformation. It is also observed that it needs to develop yield functions to model yielding behaviour under complex loading conditions.

This paper compares artificial intelligence (AI) methods to predict mechanical properties of sheet metal in stamping processes. The deviation of the mechanical properties of each blank leads to unpredicted failures in stamping processes, such as fracture and spring back. The research team of this paper has been building a real time control system for stamping process in a smart factory. In order to facilitate that, it is necessary to predict the mechanical properties of each blank with non-destructive testing. The regression models based on the linear algebraic scheme have traditionally brought reliable results in terms of matching the measured non-destructive testing values to the mechanical properties. With a parallel to algebraic regression models, in recent studies on various domains, AI models have been adopted to improve the accuracy of the end-results and effectiveness of the models. This paper discusses the applicability of AI models for predicting the mechanical properties based on the eddy-current non-destructive testing method. For the study, 6 input features are collected through the eddy-current non-destructive testing to map eddy-current input data to mechanical properties of the blank. Yield stress and uniform elongation were predicted by using five AI methods, i.e., regularized linear regression, support vector regularized linear regression, support vector regression, multi-layer neural network, random forest regression, and gradient boosting regression were compared. The model performance, validated with 20% of test data that are intact during the training phase, is the main discussion point of this paper. Future works to improve the predictive accuracy of AI models is also discussed.

The ductile fracture behavior of anisotropic materials was modeled by the uncoupled ductile fracture criterion. The ductile fracture model was concerned with the micro mechanisms of void nucleation, void growth, and evolution of void coalescence. The advance plastic model of Hill'48 non-associated flow rule is considered in proposed ductile fracture criterion. A series of basic fracture testing covering a wide range of stress state for aluminum alloy 6016-AC200 are carried out. Strain field on the surface of specimens is captured by the non-contact measurement DIC system. The fracture locus constructed using the proposed criterion is close to the experimental data points over a wide stress triaxiality range from negative to intermediate and high stress triaxiality. Then, a square cup drawing test is conducted in order to verify efficiency of the proposed fracture criterion in predicting fracture behavior of metal sheet. The results indicate that the proposed ductile fracture criterion can be utilized for predicting initial fracture in sheet metal forming.

This paper presents a local heat-treatment method to improve the local formability for stamping and trimming processes by using focused infrared (IR) rays. Although the demand of ultra-high strength steel (UHSS) has been increasing in order to reduce car weights, the low formability of the UHSS has been a limitation. In order to resolve the formability problem, heat treatment methods have been often studied, and the authors also have developed a local heat treatment method by using focused infrared (IR) rays for a decade. The difference of this method from the author's previous works is that the new heat treatment is completed by the supplier. The supplier of the materials completes the local heat treatment on target areas of blanks where to improve the local formability, then stamping companies can conduct cold forming and trimming. The target material of this paper is a 1.5GPa steel, CR 1480M steel supplied by POSCO. This paper discusses the local formability change caused by the presented IR heat treatment.

Functional integration and lightweight construction pose increasing demands on manufacturing processes and require innovative approaches. In this context, sheet-bulk metal forming combines the advantages of conventional sheet and bulk forming processes expanding the limitations of these particular processes. However, the intended three-dimensional material flow contains major challenges regarding the material flow control. To enhance material flow control and part quality the application of process-adapted semi-finished products is an expedient approach for sheet-bulk metal forming processes. So-called Tailored Blanks have a process-adapted sheet thickness profile or combine different mechanical properties within a single blank and were applied in the 1980s for the first time. Since then tailored approaches gained in importance and find broad application in research and industry. At present, various technologies are used to manufacture Tailored Blanks. The Tailored Blanks applied in sheet-bulk metal forming are often manufactured by sheet-bulk metal forming processes themselves. The achievable gradient in thickness depends on various factors but is eventually constraint by the material volume of the initial blank. In this regard, Additive Manufacturing offers new possibilities to overcome those limitations and to expand the limits of sheet-bulk metal forming processes further. Additional material can be allocated with high geometric flexibility. Besides the allocation of additional material, this technology allows the manufacturing of discrete functional elements within the production of Tailored Blanks. In this investigation, Tailored Blanks made out of stainless steel are produced using sheet material and additively manufactured elements. These Tailored Blanks are processed in a deep drawing and a subsequent upsetting operation to manufacture a functional component with an external gearing. Conventional sheet material is also processed to compare the resulting part properties and to evaluate the different semi-finished product strategies. For this purpose, an analysis of geometrical as well as mechanical Tailored Blank properties is conducted. Furthermore, the part properties after deep drawing and upsetting are analysed. The investigation evaluates the potential for Tailored Blanks consisting of sheet metal with additively manufactured elements in sheet-bulk metal forming and conclusively points out the need for further research in this field.

Modern progressive dies are increasingly equipped with integrated sensors and actuators, thus enabling an improvement of the manufacturing process and an inline monitoring of the component quality. The design of the tooling is determined at an early stage and is largely based on the desired product and the specific production stages such as blanking, forming and punching. Nevertheless, malfunctions such as strip vibrations cannot always be foreseen and may occur during operation time, which may lead to reduced component quality and in the worst case to component collision or a damage of the tooling. Therefore, complex reworking or a reduction of the stroke rate and thus lower output quantity are often necessary for a reliable and stable process. Strip vibrations are often caused by the highly dynamic transportation of the strip in the tool and can have various causes. Among others, passive tooling components such as spring-loaded, hard stop limited strip lifter can be the cause of such vibrations. Strip lifter are always necessary when three-dimensional components are produced and have to be lifted out of the die for the feeding phase. The feeding phase takes place between two strokes of the stamping process. This work is aiming for a control strategy to suppress strip vibrations in various progressive die stamping processes based on closed-loop controlled active strip lifter. These strip lifter combine the spring-loaded passive standard strip lifter with an additional PID-controlled actuator. Taking the dynamics of the flexible strip during operation into account, a Finite Element Analysis (FEA) model of a progressive die tooling system is created. For the design of the control algorithm, the FEA model is connected to an environment for model-based design in a co-simulation. This approach allows modelling the influence of arbitrary control parameter settings on the movement curve of the strip, aiming for an increased stroke rate.

In this work, nonlinear forming limit curves were developed through a systematic combination of both experimental and numerical tools. FLCs derived from both intact and pre-stretched DP440 high-strength steel (HSS) sheet were involved. First of all, prepared sheet specimens were, in compliance with the Marciniak test procedure, in-plane pre-stretched in either of the following three directions, uniaxial tension, plane strain and biaxial, at varying strain levels. Afterwards, each distinguishingly pre-strained specimen was post-strained until fracture following the Nakajima test guideline. The so-called displacement function could be determined out of these experimental data. An individual nonlinear IFU-FLCs was approximated with the help of the base linear FLC, the formulated displacement function and a unique simulated strain path collectively gathered from the local fracture zone. Two yield models, Hill'48 and Yld2000- 2d, were tried during strain-path calculations to observe how different yield models would alter the paths and thus developed nonlinear FLCs. From the study, notable variation was detected. In the end, validity of such nonlinear FLCs were proven through a complex-shaped industrial stamping part. The IFU-FLC noticeably-better defined forming limits of parts experiencing nonlinear strain paths than the conventional one.

Prediction accuracy of the two well-known anisotropic yield criteria, namely, Hill'48 and Yld2004-18p, on plastic deformation of AA5052-O sheet aluminium alloy experiencing compound stress was profoundly examined and contrasted in this work. To obtain the flow behaviour and anisotropy of the test material in varying loading directions, both uniaxial tensile and balanced-biaxial tests were conducted in the corresponding directions. Furthermore, a disc-compression test was carried out to reveal the biaxial r-value. Those gained mechanical properties could therefore be used to determine the material constants required by each yield criterion. The standard conical-punch hole expansion test was chosen here to impose complex stress on the sheet specimen. Forming parameters such as the punch load, stroke paths, hole expansion ratio, sheet thickness profile around the hole circumference and flange height were all monitored and investigated. It was shown that the results from the Yld2004-18p model with an exponent of eight apparently better agreed with the experimental data than those from the Hill'48 one. A conclusion could be drawn here that the yield criterion significantly affected the accuracy of the predicted deformation behaviour of the investigated aluminium alloy grade AA5052-O nevertheless in dependence on the degree of material anisotropy.

Stretch bending is a widely used to form long parts with a definite cross-section shape. The forming path is an effective way to eliminate the defects such as springback, distortion when the material and the die shape are determined. In this study, the stretch bending path is represented by a curve function of bending angle and stretch length. To simplify the path optimization problem, the path is represented by 4 parameters, including a newly introduced parameter of total stretch. Based on the batch simulation results, it is found that the axial springback and cross section distortion are primarily controlled by total stretch, bending angle and pre-stretch proportion. When the total stretch is determined, the different distribution schemes for pre-stretch, bending stretch and post-stretch in total stretch have a similar effect on forming quality except the schemes with extremely large proportions of pre-stretch. In most cases, with the total stretch length in growth, the axial springback decreases dramatically at first and then stays constant while the section distortion continues to rise, indicating that the optimal forming path is around the elbow point in the relationship curve of total stretch and springback angle. The optimal path is verified by experiments. It is shown that for a workpiece with determined material and die shape, the optimal path to minimize axial rebound and section distortion simultaneously does exit and the study provide an effective way to obtain it.

The hot-rolled dual-phase steel 700DP was developed to reduce weight and improve safety in automobile industry. However, strain path changes inevitably occur for most of components during sheet forming process. Therefore, it's valuable to predict the cracking accurately under nonlinear strain paths for the application of 700DP steel with finite element method (FEM). In this paper, the mechanical properties of hot-rolled dual-phase steel 580DP and 700DP were studied with tensile test. Nakazima test and two-step nonlinear loading test were used to establish the modelling with the method of equivalent plastic strain-based FLC (ep-FLC). Furthermore, the ep-FLC was applied to predict the disc forming result of three-stage process, which agreed well with the experimental results for both 580DP and 700DP compared with conventional FLC.

In a previous investigation, 3D-printed solid and topology optimized semi-industrial tools for forming and trimming of 2-mm thick hot-dip galvanized DP600 were certified. This certification required 50,000 strokes in U-bend forming and 100,000 strokes in trimming/cutting/blanking. The present paper focuses on the tool wear, the U-bend sheet surfaces, the shear and fracture zone lengths in trimming, and the punch forces in this certification. The 3D-printed tools behave as conventional tools do. Although small, there seems to be a difference in wear at the profile radius between the solid and topology optimized U-bending tool halves 3D-printed in maraging steel DIN1.2709.

Further increasing requirements to the quality of car body components represent a huge challenge for the process planner. Springback effects are causing dimensional deviations of the car body components from their respective target geometry. Both elastic membrane and bending stresses result in global and regional distortions and deflections. In addition to these effects in particular the membrane stresses furthermore result in a contraction of the drawshell with the effect that the drawshell is smaller than the tool after drawing. In order to compensate this contraction the drawing tool has to be increased. This process is known as the so-called scaling. An appropriate scaling should result in a contracted part which has the same surface area and unwound lengths as the target geometry. In case of deviations of the unwound lengths unwanted plastic deformations of the part in subsequent operations have to be expected and the part's assembly dimensions will differ from the target geometry which can cause significant quality issues in the car body. Today the scaling is usually done by increasing the tool with a unique global scaling factor or with different scaling factors for different directions. However, since the contraction varies along the part surface in reality the results are only suboptimal. Here a new scaling approach is being presented which uses the simulation results to scale the tool surfaces locally. By doing so the locally different contractions can be better compensated.

Mixed mode bending (MMB) test associated with measurement of crack tip opening displacement (CTOD) is a practical method to experimentally evaluate mixed-mode cohesive zone model (CZM) properties of polymer layers in various mixed-mode ratios. However, it has an obvious limitation that does not allow for plastic bending of adherends during the tests, making it difficult to evaluate the polymer layer inserted in the sound deadening laminated sheet (SDLS) comprising thin metal sheets which are vulnerable to the plastic bending. This paper provides an experimental technique to suppress the plastic bending of the thin metal sheets during the MMB test. Supporting tools were designed, and bonded to the both outer surfaces of the SDLS. Until delamination occurred, the evolution of fracture toughness (G) was evaluated, and the CTOD was measured in the stereo image using a digital image correlation system. Finally, the mixed-mode CZM properties was successfully established based on the differentiation of the relation between the evolution of G and CTOD.

This paper deals with the prediction accuracy of edge crack of automotive high-strength thick steel sheets according to the numerical simulation method. The mechanical tests under tension, tension/compression and loading-unloading conditions were performed to evaluate anisotropy, hardening behaviour and elastic modulus change, respectively. Edge stretchability was evaluated by the HER(hole expansion ratio) test according to the hole processing method. Based on the measured data, three simulation methods were constructed with respect to the material model and the element type. The half dome test was selected for verifying edge crack predictability according to the simulation method. The simulation results show that the pressure effect along the thickness direction has significant effect on prediction accuracy of edge crack for high-strength thick steel sheets used for automotive chassis parts.

To investigate the mechanical properties of high strength Al alloy, the AA2024 was selected and conducted the uniaxial tensile tests at room temperature under different loading modes, namely pure quasi-static (QSD) loading mode, pure dynamic loading mode and quasi-static-dynamic loading mode. The mechanical properties such as yield stress (YS), ultimate tensile stress (UTS), total elongation and strain rate sensitivity were compared. The results shown that the UTS under QSD loading mode is larger than that of quasi-static and pure dynamic loading mode, and dependent on prestrain levels. However, the YS at pure dynamic loading mode is less than that of QSD loading mode at prestrain of 10% but larger than that of prestrain of 6%. In addition, the QSD loading mode would further improve the total elongation of AA2024 compared to the elongation enhancement under pure dynamic loading mode. Finally, the variation of strain rate sensitivity under different loading modes was analyzed to explain the evolution of tensile properties of AA2024.

Sheet metal forming is a process widely used in the manufacturing industry. There are numerous sheet metal forming processes to evaluate and understand the formability. Among all formability tests, the basic formability can be formulated through tensile tests and followed with specialized tests. In the present paper, the formability of AA 6023-T6 sheet of 2mm thickness by modelling for stretching test namely limit dome height (LDH) test was performed using PAM STAMP 2G a commercial finite element software. For the simulation, input mechanical properties like yield strength (c), material strength coefficient (K), strain hardening exponent (n), plastic strain ratio (R) etc., were considered from the existing literature. For the simulation, two different conditioned sheet such as at room temperature and annealed sheet at 400°C. For all the simulations, four strain paths 100 × 200mm, 125 × 200mm, 150 × 200mm, 175 × 200mm and 200 × 200 mm were taken. Results are drawn based on the three localized necking criteria namely the effective strain rate-based criterion (ESRC – R1), major strain rate-based criterion (MSRC –R2), thickness strain rate-based criterion (TSRC – R3). Form the obtained results, forming limit diagrams are developed for the both condition of sheet metal. It is observed that, formability of AA 6023-T6 sheet in-plane condition (i.e. 100x100 mm) annealed sheet at 400°C is shown better forming whereas in bi-axial condition (i.e. 200x200 mm) got reduced compared to room temperature sheet. The same phenomenon is noted in all the necking criteria too.

Cyclic loading-unloading uniaxial tension experiments were performed for AZ31B magnesium sheets. The inelastic strain recovery behavior of the AZ31B sheets unloaded under different plastic strain was investigated. An interesting phenomenon on the elastic modulus change was presented. A new elastic modulus model was presented to characterize the inelastic strain recovery of the AZ31B sheets. Unlike the available models, this elastic modulus model takes both the instant stress and the plastic strain as the variables. The new elastic modulus model was incorporated into the ABAQUS software using the user subroutines. Validation of the subroutines and springback simulations were conducted with the presented elastic modulus models. The results showed that the inelastic strain recovery of the AZ31B sheets can reach 24% of the total strain recovery. The instant elastic modulus model can predict a perfect unloading path. The new elastic modulus model works better than the constant elastic modulus.

The onset of ductile fracture can be described by a simple damage parameter, which accounts for the accumulation of damage under arbitrary deformation. The damage parameter is usually given as integral form of strain over loading history which also has been described as a function of stress state. While the evolution of strain field can be experimentally determined using digital image correlation, experimental measurement of the multiaxial stress state is still a challenging task. Therefore, various uncoupled fracture criteria rely on stress states calculated from phenomenological plasticity model. As a result, the number of mechanical tests required to calibrate the fracture criterion may significantly increase when an anisotropic constitutive model is used. We propose an alternative approach on the basis of a mean field crystal plasticity (VPSC) model, which accounts for the microstructural features such as slip system, crystallographic texture and its evolution. While stress fields can be obtained from the use of full-field crystal plasticity framework, the proposed method utilizes the mean field crystal plasticity framework and repeat the stress estimation on various spatially resolved locations to which DIC technique provides strain history. The repeated VPSC calculations at various locations efficiently provide the map of stress evolution. The resulting map of spatially resolved stress response is further validated by comparing with the bulge stress strain curves.

Flexible roll forming enables the forming of complex automotive components from Advanced High Strength Steels (AHSS) and Ultra High Strength Steel (UHSS) but common shape defects such as end flare and springback occur and need to be compensated. In flexible roll forming a pre-cut blank is fed through a set of rolls which are computer numerically controlled (CNC) to follow the 3D contour of the part. The company, dataM Sheet Metal Solutions, have established a new 3D flexible roll forming facility at Deakin University which allows the manufacture of flexible roll formed prototypes.

In this study, Deakin's flexible roll forming facility is employed to implement an approach to overcome springback and end flare defects in a flexible roll formed automotive component of variable width. To do this, the flexible roll forming process is simulated using the commercial software package Copra FEA. Non-uniform springback due to end flare is observed and a variable overbending approach developed and implemented to overcome this defect. Experimental validation is performed on Deakin's flexible roll forming facility and the results suggest that the flexible overbending approach can be used to control final shape in flexibly roll formed components.

Self-piercing riveting (SPR) is a high-speed fastening process that can join similar and dissimilar sheet materials without the need for pre-processing such as drilling or punching. During SPR processes, two overlapping sheets are joined by a rivet. The upper sheet is punched first by the rivet and then the lower sheet is deformed between the rivet and the die, creating a mechanical interlock. In this study, self-piercing riveting of aluminum alloy and carbon fiber reinforced polymer composites (CFRP) sheets was analysed using finite element simulations. For the finite element simulation of SPR processes, the orthogonal elasticity, the fracture model, and the cohesive zone model were used for describing the behaviour of CFRP. For validation of the composite material model, the punching process of CFRP was performed and the results were compared with FE predictions. The SPR process of the aluminum alloy and CFRP was simulated numerically and the performance of the joint was evaluated.

Stress-strain relationships are used in FEM analysis. After uniform deformation, they are simply extended values expected from ones measured without experiment. The purpose of this research was to understand the stress-strain relationship in the high strain region after uniform deformation. We investigated a method for understanding this relationship in high strength materials using a simple shear test. The relationship in this region was calculated by determining a constant parameter κ as the coefficient between equivalent stresses and shear stresses. Using this constant, the relationship after uniform deformation was extended from the result of a stretching test. This method is called the κ method. In the case of high-strength materials, it is difficult to avoid rotating the specimen under a high testing load. Because of this problem, we designed the shape of the specimen so that it had two symmetrical deformation areas. The notch shapes for the deformation areas on the specimen were designed to reduce the load and provide a strong gripping force. In terms of shortening the deformation region, it is difficult to ignore a non-uniform deformation. To reduce the influence of non-uniform deformations, the effect of the R shape on the notch end was evaluated. The uniformity of two specimens with R1.5 mm and R7.0 mm on the notch end was examined by FEM. The values of κ, which were determined from the shear stress and shear strains based on the results of FEM, depended on the equivalent strain. In the case of R7.0 mm, the κ results were almost constant, and this was a suitable result for applying the κ method. The change in κ was affected by the non-proportional stress ratio, the stress error of the observed shear stress, and the strain error of observed shear strain. In the case of R7.0 mm, it was found that there was an advantage for the constant stress error in the higher strain region. Using R7.0 mm and R1.5 mm specimen shapes, we applied a shear test with two kinds of high-tensile steels and aluminum materials. From experimental results, the values for κ on R7.0 mm were also more constant than on R1.5 mm in the higher strain region. A high stress-strain relationship after uniform deformation was obtained in high-tensile steels and aluminum materials. This research could therefore be used to understand the stress-strain relationship in the high strain region of high strength materials.

In the present work, an advanced CDM model considering stress triaxiality and Lode angle effect is proposed. The framework of thermodynamics of irreversible processes with state variables is used to build the constitutive equations accounting for strong and full coupling between all the dissipative phenomena. The proposed model is implemented into Finite Element (FE) code ABAQUS/Explicit via a user material subroutine (VUMAT). A detailed parametric study with various values of the new material parameters is conducted in order to show the predictive capability of the proposed model. Applications to sheet metal forming simulation have been performed to validate the damage prediction capability of the proposed model, and the numerical simulation results are analysed and discussed.

This study presents a systematic methodology that used to identify the subsequent yield surfaces of a pure titanium grade 1 sheet at different levels of equivalent plastic work. Several experimental tests including uniaxial tensile tests (UT), hydraulic bulge test (BT), simple shear tests (SS), and uniaxial compressive tests (UC) have been conducted for samples prepared in different orientations to achieve a comprehensive experimental data of yielding behaviors observed in different forming modes. Under the condition of equivalent plastic work, the yielding behaviors are characterized and normalized to clarify the distortional hardening behavior and strength different effect for the tested material. It is seen that the yielding surface of the tested material distorts largely during plastic deformations and approaches to its final shape at an equivalent plastic work value of 40 MPa. Under the plane-stress assumption, experimental data obtained from on-axis tests (UT, BT, UC) are used to calibrate three constitutive models: Yld2k, CPB06, and CB04. Finite element analyses for a simple shear test have been performed in Abaqus/explicit software to validate the developed material models. In conclusion, the CPB06 model provides the best prediction for plastic yielding behaviors of the tested material.

Finite element (FE) simulations are very important in the field of automotive, aerospace and defense to evaluate crashworthiness of high strength reinforcing parts. Reliable material properties at high strain rates should be used for the FE simulation to acquire accurate simulation results. However, at high strain rates, it is difficult to obtain accurate stress-strain data because reasonable load data is not easily achieved from the experiments due to ringing problem caused by inertia effect. In this study, the virtual fields method (VFM) which is one of the inverse methods suggests another possibility of identifying hardening properties by utilizing acceleration data from the experiments without using load data. In the current study, a methodology is introduced for this purpose. The minimum magnitude of acceleration necessary to retrieve the hardening properties at high strain rate testing is investigated numerically and experimentally. In addition, a new type of high strain rate testing equipment, impact frame high speed tester (IFHS) is described. Various aspects allowing an increase of the acceleration magnitude in the IFHS are discussed for an optimum application of the methodology. Lastly derived hardening properties with the acceleration by the VFM are discussed.

Study of anisotropic deformation behavior of a material plays a crucial role in optimizing hot working process parameters and trustworthy Finite Element (FE) analysis in sheet metal forming processes. In this work, Khan–Huang–Liang (KHL) phenomenological based constitutive model and anisotropic yield criteria has been formulated for Inconel 718 alloy. Firstly, uniaxial tensile tests have been conducted at different temperatures (room temperature -700°C) and slow strain rates (0.0001-0.1s^-1) conditions. KHL constitutive model has been formulated and validated with experimental flow stress data. The prediction capability of the model is evaluated based on correlation coefficient (R), average absolute error, AAE (Δ) and its standard deviation (s). Subsequently, anisotropic yielding behavior of Inconel 718 alloy is predicted based on KHL yield criterion. Anisotropic coefficient (Lankford parameters) and tension compression asymmetry parameters have been calculated experimentally. The prediction capability of KHL yield criterion is analyzed based on yield locus, yield stress variation and anisotropic coefficient variation. The quality index of performance, namely global accuracy index (β) is evaluated. Further, Finite Element (FE) analysis has been carried out for deep drawing of Inconel 718 alloy using commercially available ABAQUS software. The developed KHL constitutive model and anisotropic yield criterion has been incorporated in FE simulation using UMAT/VUMAT code. The FE results are validated with experimental deep drawn cups at different process conditions.

Free-bending is a novel bending technology suitable for tube and profile bending. It can achieve accurate bending of tubes or profiles with different bending radii without die. In current study, a new theoretical analysis of a spiral tube formed by 3D free-bending is proposed to reveal the relationship between the spiral tube and the parameters of free-bending forming technology. The results obtained from the proposed theoretical analysis were compared with those achieved from FE modelling and experimentation. The maximum error of forming was less than 7%, which verified the accuracy and the reliability of the proposed theoretical model. The forming limits of the spiral tubes based on 3D free-bending were explored. It was noticed that the spiral diameter was mainly influenced by the eccentricity and the distance between the center of the bending die and the front end of the guide mechanism in the Z–direction (distance A). Furthermore, the spiral diameter is inversely proportional to the eccentricity, and directly proportional to the distance A. In addition, the screw pitch is mainly influenced, and directly proportional to the spiral diameter and the single spiral circle swing angle of the spherical bearing.

In this work, the friction coefficient between the punch and the specimen during the Nakazima tests is determined by comparing the experimental and the numerical strain paths. Afterwards, the FLCs of the Al-Mg-Li alloy sheet are predicted with the modified M-K model, which considers the through-thickness normal stress and the friction through introducing the expressions of the normal stress and the friction stress into M-K model. Finally, the predicted FLCs of the sheet are compared with the forming limits determined by Nakazima tests.

Magnesium and its alloys are attractive structural light materials because they have a high spe-cific strength and high specific stiffness. That is why their usage in high performance automotive and aerospace applications has been increased. However, their high anisotropy and low ductility could bring to forming failure, to avoid these a better accuracy for metal forming simulation is re-quired. The identification of macroscopic parameters is expensive due to the experiments needed to find them, the use of multi-scale approach allows to find parameters for the model through easier experiments. A finite element polycrystal method, based on a rate independent polycrystal plasticity model, is implemented in order to perform the material testing of a cast magnesium alloy AZ31B. The parameters of the model are adjusted to better fit the experimental data through a trial and error process.

The analysis of bend fracture limits of sheet metals is important for the automotive industry. For high and mid-ductility materials, current methods are often unreliable due to the difficultly of detecting the initiation of a crack on the outer surface of the bend. Sophisticated techniques that rely on the strain evolution in the material are capable to detect the cracks and establish the bend limits using digital image correlation (DIC) strain measurement systems. However, these techniques are complex and sometimes impractical for industrial use. This study uses a simplified bend test technique that uses optical image processing to detect crack initiation to then determine the bend limits of aluminum alloys using a modified angled line method (MALM). The findings of this study indicate that the optical method is inconsistent in detecting the crack initiation and likely to result in a large deviation in detected the bend limits.

Hot stamping process is used to produce automotive components that give a high strength-to-weight ratio, therefore it reduces fuel consumption and improves crashworthiness. Hot stamping is a non-isothermal process where forming and quenching takes place simultaneously. One of the major factors affecting the mechanical properties of components is the cooling rate which is controlled by the cooling systems. Direct cooling and indirect cooling (cooling channel) are the two cooling methods used in this process. In direct cooling, water is used as the cooling medium that gives higher cooling rate, however it induces large residual stress in the final component. In indirect cooling, cooling channels are used in die and punch for cooling purpose. In indirect cooling system residual stresses are not developed, but thinning occurs in wall region of the blank due to inhomogeneous contact between blank and die. Therefore, in this work, a new method has been proposed with the combination of direct and indirect cooling. In this combined cooling method, different coolant like spray water, air mist and compressed air was used for direct cooling. Simulation of the proposed approach was done by using PAMSTAMP software. The effect of the combined cooling system with various cooling medium was investigated and it was found that the air mist cooling with indirect cooling system resulted in 7% improvement in thickness distribution and 20% improvement in temperature distribution compared to the conventional method.

In this work, the virtual fields method was applied to identify the anisotropic plastic constitutive parameters for the wrought 2024 aluminium alloy. First, low cyclic tension-compression tests were conducted for the specifically designed aluminium alloy specimens on a hydraulic universal testing machine and the full-field deformations measured by digital image correlation. Then, the measured experimental data were used to simultaneously identify the constitutive parameters of the Hill 1948 yield criterion and the nonlinear kinematic hardening model based on the virtual fields method. Reasonable identified parameters were obtained. The fitting curves of the virtual work during the minimization process verify the reliability of the identification results.

The forming process using ultra high strength steel (UHSS) may cause early damage than the expected lifetime of the tool surface due to the high reaction force of the material compared to the conventional steel sheet. This leads to the increase in the cost of maintenance of the tool and the decrease in the product productivity and quality. In this study, the sliding wear behavior of the titanium carbonitride (TiCN) physical vapor deposition (PVD) coated pin against UHSS was quantitatively evaluated from the pin-on-disk test based on the Taguchi method. S/N ratio and ANOVA are used to evaluate the sensitivity of process parameters affecting on the pin wear depth. Finally, a wear prediction model for the TiCN coated pin against UHSS was constructed by the power law equation.

In the case of complex forming process that receives a secondary loading after the primary loading, it is necessary to appropriately represent the work-hardening state according to the forming history. Namely, it is necessary to accurately model complex forming process including reversal loading (Bauschinger effect) and orthogonal loading (cross-hardening). In this study, a composite anisotropic hardening expression based on the crystallographic structure using the finite element polycrystal model is proposed. Numerical investigations are conducted to consider the effectiveness of the proposed model. In which, an alternation rate is introduced to capture the activity of the slip systems to support the idea behind the proposed model. In addition, flow curves of the secondary loading are produced by the proposed model, which is compared with the conventional model.

Eddy current (EC) testing is one of the most extensively used NDE techniques in the automotive industry for automatic in-line inspection of ferrous materials such as advanced high strength steels (AHSS). In addition, shearing is a very common forming operation in the automotive industry. With the increase of shearing clearance, the sheared-edge experiences significant work-hardening that normally decreases the formability of the sheared edge. This paper introduces a novel and real-time monitoring NDE method based on the EC sensor to characterize variations in shear edge quality for two different grade advanced high strength steels, DP780 and 980GEN3. The developed NDE method was applied to scan the edges sheared at various clearances between 5 and 25% of the material thickness. The NDE signal received was correlated with pre-straining introduced during the shearing process at various clearances. Microhardness measurements were taken to compare the trends obtained from the NDE tool with the hardness values. To evaluate the edge formability, half-specimen dome testing (HSDT) was conducted for the edges sheared at various clearances. A digital image correlation (DIC) system was used to measure failure strain during the HSDT. The failure strain of sheared edges was correlated with the NDE measurements for each clearance to assess the application of an NDE measurement in determining edge quality. The NDE method can potentially be used to inspect the sheared edge quality for sheet metal stamping process.

In this study, the effect of shearing speed on edge formability was investigated with two different grades of advanced high strength steel (AHSS), DP780 and GEN3-980. A straight blanking tool with adjustable shear clearance was used to shear the selected materials in two different type presses, a high-speed pneumatic press, and a mechanical servo-press. The four different shearing speeds from 0.01 meter per second up to 1.5 meters per second were realized and measured. The different shearing speed conditions result in different edge quality and work hardening levels. The half specimen dome test and micro-hardness analysis were conducted to evaluate the edge formability. The samples sheared with a pneumatic press at a higher speed showed significantly better edge quality, less hardness increase, and smoother sheared edge than the samples sheared with the servo press. The developed high-speed shearing condition has a very good potential to shear AHSS without creating significant damage and work hardening on the sheared edge, providing further advantages for the post stamping operations.

With the current increasing emphasis on issues of energy saving, emission reduction, driver and passenger safety in automotive industry, lightweight bodywork is an effective measure to reduce vehicle energy consumption. And also, high-strength bodywork structure can improve automobile safety performance because of its high impact absorption energy, and high fatigue resistance. In sheet metal forming, springback is a main defect with using high strength steel. Aiming to investigate sheet metal springback phenomenon for high-strength steel, a multi-funtional springback test machine is built. The testing machine can perform four types of springback tests: V-bend, U-bend, stretchbend and drawbend, and all these tests have been carried out to investgate the effect of springback factor in sheet metal forming processes for example forming speed. The research found that the springback angle of U-bend and drawbend increase with enlarging speed, however, V-bend and stretchbend show the opposite trend.

The predominant mode of failure in stamping applications is still splitting due to localized necking. This kind of failure is very effectively captured by the forming limit curve, especially if nonlinear deformation effects are modelled. With the sharp increase in low-ductility (weight efficient) materials, direct fracture phenomena (surface cracks, edge cracks, shear cracks, etc.) increasingly enter the daily industrial stamping practice, especially in follow operations. Contrary to the classical FLC analysis, there exists nowadays no industry-wide standardization for characterizing these phenomena. This drives up modelling costs and reduces the expected improvement in the overall modelling uncertainty. It therefore becomes crucial to understand and detect the circumstances in which an advanced fracture characterization is necessary, such that limited resources can be effectively allocated. The present contribution aims to propose such a workflow. Firstly a reasonable lower bound for the fracture limits is estimated based on theoretical considerations and evidence from scientific literature. This estimation is used to conduct a first analysis step which is used to categorize the process based on whether a standard modelling approach is sufficient to deliver the required accuracy. A second step of accurate fracture identification is therefore only recommended for the subset of geometries with particularly high risk. The approach is tested on data for AA6016 and DP980 from literature and validated based on published experimental results.

The accuracy of the simulation results in terms of metal sheet forming strongly depends on the capability of modelling the anisotropic material behaviour. In addition, predictive capabilities of the models are strongly influenced by the way how the constitutive model parameters are calibrated. Macroscopic models lean towards to become more complex in order to map the material behaviour more precisely. As consequence the amount and complexity of the experiments is increasing as well. In addition, it is well known that, some of the experiments, for example the equibiaxial compression test, are difficult to perform and therefore, a reasonable coupling of crystal plasticity (CP) modelling and macroscopic models is proposed. It is worth to mention that, in the domain of CP, arbitrary load cases are possible and therefore, any stress ratio of the yield criterion can be used for calibration. Prediction of anisotropic material behaviour of AA6016-T4 and DC05 sheets based on CP simulations were previously presented and compared with the macroscopic Yld2000-2d model. Their data set is now used for the calibration of the parameters of the macroscopic model, where in contrast to the classical procedure, the exponent of the yield locus is defined as a fitting parameter. The strain distributions predicted by the models have been compared with DIC-measurements of Nakajima samples. The predictive capabilities of the CP-based fitting procedure, compared to the classical fitting, are highlighted. Additionally, a comparison of the strain distribution prediction between all model variants is performed on a cruciform shaped deep drawing part. It underlines the importance of the correct prediction of the yield normal, as it is given by the crystal plasticity computation.

Series production of sheet metal components in press shops is permanently subjected to an increasing pressure in time, cost and quality. Varying sheet metal properties due to batch fluctuations and changing process conditions lead to an increase of try out time phase and scrap rate, resulting in a demand for adaptive control strategies for deep drawing. Today, these chal-lenges are met by integrating sensors and actuators into tool structure which measure and control part quality, mainly through blank draw-in. However, such tool-based control systems require a complex and cost-intensive modification of the existing die or press technology. Against this background, the active adjustment of blank position prior the deep drawing process realized with an intelligent transfer and positioning system as a promising approach towards economic and technical aspects. This paper deals with blank positioning and its sensitivity to quality-related failures of deep drawn sheet metal components like splits and wrinkles. Therefore, a numerical study was conducted on a deep drawing process of an exemplary structural part geometry, wherein a typical fluctuation of material parameters and variation of local friction conditions was simulated. Subsequently, correlations between process disturbance and blank draw-in were elaborated, thus using local draw-in values as controlled variables for a closed loop system. Sim-ulation results showed that the manipulating parameter, i.e. the blank position prior deep draw-ing, reveals a significant influence on the draw-in and therefore on the component s quality. An essential finding of this study is the numerical proof of concept for this new deep drawing control strategy, demonstrated for different process conditions. Finally, disturbances in the deep drawing process considered could be successfully controlled by adapting the blank position.

Many different models have been published to predict failure after non-proportional load paths. Most of those models are phenomenological and heuristical models. They require a profound knowledge about the material. Examples are the enhanced Modified Maximum Force Criterion (eMMFC), the Polar Effective Plastic Strain-model (PEPS) or the Generalized Forming Limit Concept (GFLC). In addition to the load path, the loading direction has a significant influence on the formability of sheet metals. The mentioned models currently neglect this influence. By extending the GFLC-model by the parameter of loading direction, this influence is taken into account. By analyzing an acceptable number of bi-linear experiments, it is possible to calibrate the proposed model for a micro-alloyed steel HC340LA. Therewith an arbitrary load path with a change in loading direction can be evaluated. The results of this contribution show the effectiveness of this approach by different experiments.

To improve the formability and productivity in cold forming of aluminum alloy, w-temper forming process was developed. In this process, the T6-temper blank is subjected to the solution heat treatment (SHT) and then quenched to achieve the w-temper. At present, the main problem of this process is that too long SHT time cannot meet the manufacturing cost. For example, the quenching time is 10 sec, while the SHT time is 30 min. In this paper, rapid solid solution of Al 7075-T6 alloy sheet was implemented in near-infrared ray(IR) heater. Applying IR, contact heater was developed instead of the traditional heater using cartridge heater. The results have shown that it is possible to accurately control the sheet temperature for solid solution temperature in short time. Also, compared to SHT using traditional heater, the rapidly heating using IR heater could make the mechanical properties of w-temper.

In Crystal Plasticity Finite Element Method (CPFEM), normally over thousands Euler angles are used. It leads to high computational cost. To efficiently solve this problem, a reduced texture approach was implemented through User MATerial Interface (UMAT). Specific material parameters including the texture information were calibrated to characterize anisotropic behavior. For the calibration, it is used the stress-strain curves and r-values along the rolling, diagonal, and transverse directions. In this study, AA 2090-T3 was modelled with the reduced texture approach by characterizing 12 parameters. Single element simulation result from the reduced texture approach shows a good agreement with the experimental data. In addition, a deep drawing simulation for AA 2090-T3 was performed. The simulation results from the reduced texture approach were compared with those from the advanced constitutive models such as Yld2000-2d and Yld2004-18p in terms of accuracy and time efficiency. It shows a great potential that the reduced texture approach based on the crystal plasticity theory could be applied to macroscopic engineering problems as an alternative solution for continuum level advanced constitutive models.

Single Point Incremental Forming (SPIF) is a flexible, novel and rapid sheet forming process to deform extra thin to the thick sheet with the help of computer numerical control unit, hemispherical tool and fixture. A tool follows a series of small incremental steps which deforms the sheet into the desired shape. Despite of recent development occurred in past, the little literature was reported which investigate the effect of various types of lubricants in SPIF process. This study aims to address the distinct type of lubricant to accomplish better surface roughness and formability of final parts while incremental forming of aluminium 5052 H32 sheets. After the forming process, the fractographies of the specimen were studied at optimal condition. The study is also extended to determine residual stresses generated due to incremental deformation and its effect on corrosion phenomenon with the environment. It has been found that the fracture zone material is likely to corrode more rapidly due to increased plastic deformation than the material at initial and middle forming zones.

This paper proposes a bending method to measure the robustness of advanced material constitutive models, which are developed aiming to precisely express material responses including the Bauschinger effect, permanent softening, etc. Such constitutive models, in general, require multiple sophisticated material tests, e.g. cyclic uniaxial tension-compression, followed by appropriate numerical optimizations to identify corresponding material model parameters. These tests, for example, intrinsically require anti-buckling measures for specimens that can lead to redundant frictional forces and biaxial effects. Besides, the strategy for the optimization of material parameters or the selection of elastic modulus model is also an important point to influence the final performances. In an effort to provide more robust reliability for springback simulation results, a wipe-bending tool enabling cyclic load reversals has been developed as an intermediate validator positioned between the role of the cyclic uniaxial tension-compression and that of the U-drawbending. Predicted springback results employing the Yoshida-Uemori model have shown that this approach can be considered as an effective way to confirm both the reliability of material model parameters and the capability of a selected constitutive model in a part development or tooling stage.

Shortened product development processes of car body components made of sheet metal combined with the upcoming lack of experts and craftsmen's in that field do challenge car manufacturers fundamentally. Required tryout time phases and manufacturing costs of complete die sets for car body panels are significantly influenced by their proper function in production and therefore by their highly developed manufacturing process. Looking in more detail, elastic deformation of costed two components and forming press behaviour under load impacts the contact areas between the sheet metal and the load specific spotted areas of the tool. Especially in deep drawing such contact areas between matrice and blank holder are strongly influenced by the sheet metal properties and the part flange behaviour in terms of thickening and thinning during the process. Modern advanced forming simulation models indeed increase precision of sheet metal forming simulation remarkably. For example, new friction models and structural modelling of forming die and forming press components do improve simulation accuracy, while still using rigid tool components. Considering tool stiffness and press deformation in the die design by generating specific blank or geometries via stiffness optimisation, elastic deformations and pressure distribution presumably can be homogenised. In this respect, this paper presents a numerical approach for minimising elastic deformations in drawing operations and thus reducing trial time in the early engineering face. Here a stiffness optimised blank holder is designed based on topology optimisation, taking into account press deformation characteristics such as used drawing pins underneath the blank holder and the drawing cushion box integrated in the press bolster. Afterwards, the new designs compared to conventionally designed die based on structural simulations. The main finding of this contribution is that new approach presented on the one hand leads to improved tool stiffness and reduced tryout effort and more robust production conditions on the other hand.

Hot stamping of aluminium-silicon (Al-Si) coated 22MnB5 steel blanks is widely used in the automotive industry to produce light yet crashworthy parts. However, the coating melts at ∼577°C and transforms into a rough intermetallic layer as iron from the base steel diffuses towards the surface. The blank surface roughness impacts the radiative properties during heating as well as weldability, paint adhesiveness, and cooling rate during forming and quenching. This study pioneers the use of laser speckle patterns, caused by the constructive and destructive interference of collimated light reflected off the blanks, to infer the evolving surface roughness of Al-Si coated steel coupons in situ. The results reveal a significant increase in surface roughness once intermetallic compounds reach the surface and that higher furnace set-points produce rougher parts.

The heating phase of hot stamping is the most time-and energy-intensive part of the process, and therefore offers significant potential to improve process efficiency. This requires a robust thermometallurgical model that predicts the blank heating profile and the austenitization progress inside a roller hearth furnace. In this paper, an IM austenitization model for 22MnB5 steel is derived using dilatometry data and evaluated against a JMAK type model using temperature profiles of coupons heated within a laboratory-scale muffle furnace and blanks heated within a roller hearth furnace. The austenitization rates predicted by the two models are then compared to the inferred austenitization processes from those temperature profiles. The result shows that the IM model has potential to capture the trend of austenitization process, especially the rapid pearlite dissolution at the beginning of the transformation, whereas the JMAK type model is too restrictive and fails in this regard.

The layer compression test is an established method for the characterization of sheet metal behavior under equibiaxial tension. Due to its ability to achieve high strains without necking and the simple experimental setup, the test is a welcome alternative to the more complex Bulge test for flow curve determination. In addition, the layer compression test provides further data for the calibration of yield loci models. However, the evaluation of the test is not trivial, since a tactile extensometer measurement incorporates the error of closing gaps between single sheet layers. Moreover, the sheet metal anisotropy is neglected. To overcome the above-mentioned drawbacks, a measurement and evaluation methodology based on the optical strain measurement of two orthogonally positioned 3D digital image correlation (DIC) systems was introduced recently. The method delivers reliable results but increases the complexity and costs of the test. Here, a simplification to this method is proposed. By the usage of only one 3D DIC system, the major in-plane strain is acquired from an online measurement. The minor in-plane strain is calculated posteriori by the consideration of the biaxial Lankford parameter. In this way, the anisotropy and thus the correct flow behavior can be studied by only one 3D DIC system. The proposed method is evaluated and discussed.

The cast and machined automotive cylinder block and the cylinder head serve to locate and hold the combustion cylinders in place, provide for cooling channels for the combustion cylinders and damp out vibrations, besides housing allied components needed for combustion. In the present work, the authors attempt to redesign the cooling system of the engine block for manufacture from sheet metal, and reduce its weight. It is proposed to have an air-cooling system around each of the combustion cylinders using a finned cooling cylinder rotating concentric with respect to the combustion cylinder with a coolant occupying the annular gap between the combustion cylinder and the cooling cylinder. The peculiar design enables hotter air to enter the combustion cylinder thereby increasing the expected thermal efficiency and power output, besides enhancing the quality of air-fuel mixing. Manufacture of the engine block from sheet metal involves embossing flow channels into thin sheets for the coolant, or machining such channels in thick sheets, folding them over, brazing the flat surfaces between channels and brazing the fin to the periphery of the cooling cylinder.

The instrumented 3-point bending test according to VDA238-100 test standard is increasingly used within the steel and automotive industry. Originally developed for aluminum hemming characterization, this bending test has been shown to be also relevant for local formability and crash foldability assessment originally of press-hardened steel grades and more recently for newly developed advanced and ultra high strength AHSS/UHSS steels grades. This instrumented bending test delivers bending load vs. bending angle curves. It is commonly assumed that material failure shortly happens beyond maximum load after a 30N load drop. The bending angle at maximum force aFmax characterizes then the bendability of the investigated material. The assumption maximum force = bending crack initiation, while being true for press-hardened grades, is in too many case not valid for steel grades with tensile strength ⩽1200MPa and cannot be universally trusted. An alternative approach is presented using passive acoustic emission sensors placed in the vicinity of the bending punch. The interpretation of such acoustic data is however quite subjective and still in trial status. Redundant crack detection systems based on load, acoustic as well as optical measurements may have to be considered together for increasing crack detection reliability within the VDA238-100 bending test specification.

There is a growing interest in correlating usual tensile testing results with edge crack sensitivity testing from punched ISO16630 hole expansion ratio HER (10mm shear cut hole, 12% clearance, conical expansion tool). A new kind of tensile local ductility parameter has been developed lately based on broken sample surface of tensile specimens after testing. Reduction in area or thickness at fracture are more sensitive than conventional fracture elongation with a 50 to 80m gage length to characterize the local ductility potential of sheet steels. A representative amount (300 different sets of samples) of cold rolled sheet steels have been tested in the tensile strength range 600-1200MPa and thickness range 1-2mm with 3 replicates in the transverse and longitudinal direction with ISO 6892-1, type 2, A₈₀ tensile samples. Correlation levels of ISO16630 HER values with conventional tensile mechanical properties such as uniform & fracture elongation, yield & tensile strength, n-& r-values or derivatives are disappointing low for the investigated AHSS grades. There is however a massive improvement in the empirical statistical correlation when using local ductility properties based on fracture area or thickness reduction measurements on broken tensile samples. Logarithmic local ductility strains correlate generally linearly with logarithmic hole expansion ratio. Logarithmic true local ductility values are proving more suitable than engineering strains for correlations. Transverse direction improves slightly the correlation quality vs. longitudinal direction. The correlation is also higher for thickness reduction in comparison to reduction of area based properties.

In order to characterise mechanical properties of materials (e.g. formability) under hot stamping conditions, significant efforts have been made to the development of the biaxial tensile testing method using cruciform specimens. However, no method for necking strain determination and no cruciform specimen design have been widely accepted. In this study, a new technique for characterising mechanical behaviour of materials under hot stamping conditions has been proposed. It includes two main parts: 1) a novel spatio-temporal method for determining necking and fracture strains, and 2) a cruciform specimen design for formability evaluation using biaxial testing method. In the first part, the theoretical base of the novel spatio-temporal method has been discussed, and the method has been validated by applying to uniaxial tensile tests on AA6082 specimens. The method has also been compared with several existing popular methods, in the determination of limit strain at onset of localised necking. It is found that the novel method has greater simplicity, stability and accuracy for the determination of localised necking strain. In the second part, a proposed cruciform specimen of AA5754 has been tested under the equi-biaxial tension, and both the necking initiation location and the strain path at the location where necking initiates, have been analysed. Furthermore, the novel spatio-temporal method has been applied to the biaxial tensile test for the determination of necking and fracture strains. The results show that the designed cruciform specimen enables to initiate fracture at the centre of the specimen and realisation of linear strain path under equi-biaxial tension.

In order to achieve the effective joining of aluminum and magnesium alloy sheets, a friction-assisted clinching is innovatively proposed to reduce material damage during the process and improve the joint quality. 2 mm thick Al5052 aluminum sheet and 2 mm thick AZ31 magnesium alloy sheet were selected as the research objects, the interaction between different forming parameters, i.e., die depth, rotational speed and rotation time, and the joint quality, i.e., the damage, neck and interlock were explored by DEFORM-3D. The influence of sheet temperature on damage and punch load is analysed, the rotational speed and rotation time have great influence on the forming joint, the die depth mainly affects the neck and interlock. Comparing the joint qualities under different forming parameter combinations, it is found that the joint quality is the best when the bottom die depth is 2.2 mm and the bottom punch is applied at 1000 rpm for 0.5 s. The research will provide a valuable reference for the realization of joining the aluminum and magnesium sheets.

In order to achieve the targeted crashworthiness performance, the application of Tailor-Welded-Blank (TWB) on the crash-relevant components has become a trend in the automotive industry. This paper aims to investigate the mechanical property and structural performance of the hot stamped TWB B-pillars. After hot stamping, the strength grade for the hard zone is 1500 MPa while the soft zone's strength grade is 590 MPa. The CAE model shows that the intrusion distance is decreased for the TWB design compared to the monolithic material B-pillar. The three point bending performance could be adjusted by changing the range of the soft zone. It is observed that the weld line has a low crack sensitivity during hot stamping. The transition zone between the hard zone and soft zone is less than 1mm. The ATOS scanning results show that no significant effect on the part's size deviation for the TWB design compared to the monolithic material B-pillar.

An advanced strategy for iterative definition of initial yielding based on planar strain distribution is presented. It is shown that full-field DIC measurement of NTR5 samples provides information on initial yielding for plane strain. The interdependency of strain increment and yield locus under assumption of associated flow allows for definition of yield parameters using a non-linear optimization scheme with LS-OPT. Pivotal for research in direction of additional support points for definition of initial yielding was the discovery that definition of yielding based only on tensile and biaxial experiments is not sufficient for aluminum alloy. Special focus was placed on the area of generalized plane strain, which is the most critical stress state. Previous publications illustrated experimental options using cruciform tension and crystal plasticity as support points in generalized plane strain. This publication introduces an additional strategy to determine data for multiaxial stress states without need of additional experiments. The iterative strategy shows promising results for definition of yielding in generalized plane strain. Additionally, it is illustrated that common yield models such as non-quadratic YLD2000-2D and free-shape Vegter are sufficiently capable to describe yielding of aluminum alloy, if their full potential is exploited. The strategy is evaluated on the basis of Nakajima strain distributions and a conclusion is drawn on applicability and predictive capabilities.

The hole expansion test has become popular since it allows to study the formability of metallic sheets, in particular the onset of necking in stretch flanging areas. The accurate prediction of strain localization and consequent fracture requires a proper description of the plastic behavior, particularly the anisotropic yield function. In the context of strain localization prediction, the yield criterion adopted plays an important role, particularly when using an associated flow rule, since the direction of the plastic strain tensor is modelled by the normal to the yield surface. In this work, the parameters of an advanced yield criterion are calibrated considering a wide set of experimental data, which includes results from uniaxial and biaxial tension tests. This enables establishing yield surfaces with similar shape in the plane defined by the stress components in the rolling and transverse directions and a null shear component in the same plane. However, their shape changes slightly when considering non-null values for that shear component. The numerical simulations of the hole expansion test demonstrate the impact of these slight differences on the thickness strain distribution and, consequently, in the instant and location of the necking.

The increasing use of advanced high-strength steels in sheet metal forming processes requires improved knowledge concerning the thermal and contact conditions. This study presents the experimental and numerical analysis of tensile and draw-bead tests, considering a dual phase steel DP780. The temperature evolution was measured in both tests using an infrared thermal camera. The presented finite element model considers both the heat generated by plastic deformation and friction, as well as the heat loss to the environment by free convention and the contact conductance. The temperature rise in the uniaxial tensile test is accurately predicted by the numerical model. Regarding the draw-bead test, the pulling force is accurately predicted but the temperature variation is overestimated, requiring further investigation.

It is well-known that the use of a conical die in cylindrical deep drawing can enhance the formability compared with conventional process. The most commonly observed failure mode in the process is fracture near the punch corner or wrinkling in the flange region. In order to obtain the limiting drawing ratio (LDR), a cylindrical deep drawing including only the conical die and flat punch is discussed based on numerical simulation in the paper. A finite element model for the deep drawing with a conical die is developed, and the accuracy of the model is verified by experiments results. The investigation shows that in the cylindrical deep drawing with a conical die, the types of failure above, which can obtain for thinner sheet materials, are obviously related to the sheet thickness and the conical angle, and the optimum conical angle is also found.

Fracture toughness has become a key property to predict the fracture performance of high strength metal sheets (edge cracking resistance, crash failure behaviour, local formability, etc.). However, the measurement of the fracture toughness of thin sheets still being challenging, mainly because of complex, expensive and time-consuming specimen preparation. In this work, an innovative tool to readily assess the fracture resistance of thin advanced high strength metal sheets is presented. The device consists of a special cutting tool (punch and die) designed to introduce sharp notches in sheet specimens through a simple shearing process. This new method avoids the need for fatigue pre-cracking procedures and allows measuring the fracture toughness of thin metal sheets with easy and cheap specimen preparation. It has been used in this work to evaluate the crack propagation resistance of four different advanced high strength steel sheets. The obtained toughness values are in good agreement with those measured with fatigue pre-cracked specimens and they show to be suitable to predict edge formability of AHSS sheets.

Chain-Die forming is an emerging gradual forming technology suitable to fabricate advanced high strength steel (AHSS) products. In this paper, a variable-width product was designed, and the feasibilities of chain-die forming were investigated with experiments. The results show that chain-die forming is capable of manufacturing AHSS variable-width products with good precision. However, warping defects were found in the chain-die formed variable-width products. Meanwhile, web-warping and flange-warping show a similar trend for the designed variable-width profile, and the maximum deviation of warping was quantitatively analyzed. The occurrences of warping defects are ascribed to the insufficient tensile or compressive deformation, and both web area and flange area of hat profile tend to rotate to compensate for the geometrically necessary longitudinal strain.

The Ni-Cr based IN625 alloy has been extensively used in critical applications of aerospace and nuclear industries. Various components have been manufactured using sheet metal forming processes. In the present work, the deep drawability of material has been examined under the influence of different process parameters such as temperature, punch speed, lubrication, and blank holding pressure using the processing window. The deep drawability has been analyzed in three different zones, namely, safe, wrinkling, and fracture. The limit drawing ratio increased by 7.12 % on increasing the temperature from 300K to 673K. The thickness distribution across the deep-drawn component plays a crucial role in examining the quality and life of it. Therefore, in this context, a detailed analysis of different process parameters over the drawn cup's thickness distribution has been done. The minimum thickness was obtained near the punch corner or the cup junction region of the deep drawn cup. The uniformity in the thickness distribution and drawing height also increased with the forming temperature and decrease in punch speed. Additionally, two different yield criteria, namely, Hill 1948 and Barlat 1989, have been analyzed and further used for the numerical analysis using the user material subroutines in Abaqus software. The results predicted using Barlat 1989 yield criterion more closely followed the experimental results as the average absolute error obtained is well within the 5% of the acceptable limit.

In sheet metal deep drawing process, the sliding contact between mold and sheet metal will cause friction and wear between sheet metal and die, resulting in tensile breaking and die wear, which will not only shorten the service life of die seriously, but also lead to quality decline and the rising costs of the forming process. In this paper, an anti-friction method using chemically etched surface texture collocated with graphene nanoparticles added lubricating oil was proposed to reduce the COF between sheet metal and die during sheet metal deep drawing process. Several surface textures with different parameters were obtained by surface chemical etching treatment on the die. The surface texture was observed by Laser Confocal Microscope. The results show that the surface texture after corrosion consists of micron-scale convex and concave, in the process of friction, when the pressure is higher than 72 MPa, the concave in the surface texture forms a closed oil concave because of plastic deformation, lubricating oil provides static pressure bearing capacity in the closed oil concave, sheet metal was separated by the oil from die contact surface, which effectively reduced the COF. In addition, the anti-friction properties ware tested, which showed that when the mass fraction of graphene nanoparticles added is 0.3%, stable deposited film in the friction contact interface can be formed, and a good filling effect of the lubricating oil in the sealed oil concave can be guaranteed, which can preserve a better integrity of the surface texture.